1. Field of the Invention
This invention relates to high powered coherent radiation sources and more particularly to an efficient, continuously tunable free electron laser which can efficiently generate high power radiation in the cenlimeter, millimeter, infrared, and optical wavelength range.
2. Description of the Prior Art
During the past ten years there has been a great dea1 of interest in using a re1ativistic e1ectron beam passing through a periodic, transverse d.c. magnetic fie1d to generate or amp1ify tunab1e coherent radiation as considered by J. M. J. Madey. U.S. Pat. No. 3,822,410 and is known as the free electron laser (FEL). In the FEL an electron accelerator provides a relativistic electron beam of energy E=.gamma.Mc.sup.2 where M is the electron rest mass. c is the speed of 1ight in vacuum. and .gamma..sup.-2 =1-v.sup.2 lc.sup.2 with v being the electron ve1ocity. The e1ectron beam passes through an injection magnet which steers the e1ectron beam such that the e1ectron beam enters a periodic, transverse, d.c. magnetic field on the axis of this magnetic field. The electron beam travels parallel to the axis of the periodic. transverse, d.c. magnetic field over the entire length of the magnetic fie1d. The periodic, transverse. d.c. magnetic field accelerates the electrons periodically in the direction transverse to the axis of the magnetic field, thereby causing the electrons to spontaneously emit polarized radiation that travels in the same direction the electron beam is traveling. The wavelength of the emitted radiation is given by ##EQU1## where ##EQU2## e is the electron charge, .lambda..sub.o is the constant period of the transverse, d.c. magnetic field, and B.sub.o is the constant amplitude of the transverse. d.c. magnetic field. The emitted radiation wavelength can be changed by either varying .gamma. or the magnetic fie1d amp1itude B.sub.o . After the e1ectrons have passed through the periodic, transverse, d.c. magnetic field, they pass through a bending magnet that steers them away from the radiation beam path. For a constant period and constant amplitude transverse d.c. magnetic field free electron laser, the conversion efficiency of electron kinetic energy into radiant energy is typically limited to values of less than one percent. in order to improve the energy conversion efficiency, it has been proposed to vary the transverse d.c. magnetic field period and/or amplitude along the length of the magnet subject to the constraint that the output radiation wavelength remain constant. (U.S. Pat. No. 3,822,410 by J. M. J. Madey; and "Free Electron Lasers With Variable Parameter Wigglers", by N. M. Kroll, P. L. Morton, and M. N. Rosenbluth. IEEE J. Quantum Electron, Vol. QE-17, No. 8, pp. 1436-1468, August 1981.) Although varying the magnetic field period and/or amplitude along its length can improve the energy conversion efficiency an order of magnitude or more over that of the constant parameter device, the small signal gain of the varying magnetic field device is typically much less than that of the constant parameter free electron laser. It is necessary to have the small signal gain as large as possible to quickly reach the saturated state where the varying magnetic field technique improves the energy conversion efficiency. If the small signal gain is not large, the device needs to be made longer so that it will reach the saturated state.
Another proposed technique for improving the energy conversion efficiency is to apply an axial d.c. electric field along the entire length of a free electron 1aser that uti1izes a constant period and amplitude periodic, transverse d.c. magnetic field. Studies have been made in this area by A. Gover, C. M. Tang, and P. Sprangle, "Design Considerations of a Compton Scattering Free Electron Laser With an Axial Electric Field", Phys. Quantum Electron., Vol. 9, pp. 795-815. 1982; H. R. Hiddleston, S. B. Segall, and G. C. Catella, "Gain Enhanced Free Electron Laser with an Electromagnetic Pump Field", Phys. Quantum Electron., Vol. 9, pp. 849-865, 1982; T. Taguchi, K. Mima. and T. Mochizuki. "Nonlinear Saturation Mechanisms and Improvement in Free Electron Lasers", Phys. Quantum Electron., Vol. 9, pp. 817-848, 1982; H. Takeda and S. B. Segall, "Amplifier Optimization Study for an FEL Wiggler with a Helical Magnetic Field and an Axial Electric Field, " IEEE Trans. Nucl. Sci., Vol. NS-30, pp 3112-3114, August 1983; A. Bhowmik, R. A. Cover, and W. A. McMullin, "Comparison of D.C. Electric Field and Tapered Wiggler Free Electron Laser Efficiency Enhancement Schemes", IEEE J. Quantum Electron., Vol QE-21, pp. 998-1006, July 1985. However, applying an axial d.c. electric field along the whole length of the free electron laser, like varying the magnetic field period and/or amplitude, has the drawback of degrading the small signal gain.